Experts say health and life expectancy gains mean it is time to rethink how we measure and define being elderly.
The Office for National Statistics team says although 65 has traditionally been seen as the start of old age, 70 could be seen as the "new 65".
That's because many people who reach this milestone birthday can still expect to live another 15 years.
Remaining life expectancy may be a better marker of old age, they say.
What age is old?
Traditionally, 65 has been taken as the entry point into old age.
For decades it has been the official retirement age for men when they can start drawing their state pension.
But working patterns are shifting and the pension age is rising for both men and women - it will reach 66 in 2020 and 67 by 2028.
The ONS team says people are living longer, healthier lives.
That means we should consider the years people have ahead of them, not just chronological age when deciding what "old" looks like.
The ONS Centre for Ageing and Demography team looked back at population data on health and life expectancy to compare trends over time.
If you take the measure of 15 years of life remaining, the average age that a person will hit this point has changed over the last century.
In 1951 men and women around the age of 60 could expect to live another 15 years. By the 1990s it had shifted to 65 and currently it's 70-year-olds who can expect another 15 years of life.
In 2057, experts predict this average age will rise again to 75.
The population's life expectancy has increased over the centuries thanks to improvements in healthcare and living conditions.
But are these extra years enjoyed in good health? Is it fair to shift the milestone of old age from 65 to 70?
Older and better?
Health by chronological (birthday) age has improved over time.
But changes in health by prospective age (how much time you have left to live) have declined in some cases and improved in others.
By the researchers' calculations, levels of poor general health for women aged 70 in 2017 were around the same as for those aged 60 in 1981, while levels of longstanding illness were similar for women aged around 64.
For men, levels of poor general health at age 70 in 2017 were around the same as for those aged 65 in 1997, while levels of limiting longstanding illness were similar for those around age 57.
So, at a population level, it would seem fair to label 70 as the new 65 (or perhaps even younger), even taking health into account.
Libby Webb, senior research manager at Age UK, said: "People at age 70 now have the same life expectancy and similar health to people aged 65 in the past so, on average, we are definitely seeing people doing better than they did in the past."
But she said most people still experienced a period of poor health at the end of their lives.
"What we are not seeing is that period getting shorter."
She said improved life expectancy meant there were now more "very old" people with quite complex care needs and that health inequalities between the richest and poorest remain.
But she said an ageing society should not be viewed as negative.
"We know that older people make really important contributions to our society both through their paid work and through their caring responsibilities and volunteering.
"Age is just a number and for different people it means different things."